Tomasz Gizbert-Studnicki (Jagiellonian University in Krakow) has posted Legal Theory's Claim to Necessity on SSRN. Here is the abstract:
Legal theories define their task and discovery of necessary and/or essential features of law. It is unclear what “necessary” and “essential” mean in this context. This paper focuses on the problem of necessity, claiming that what is meant is metaphysical necessity. The clue to metaphysical necessity is the ideal secondary conceivability. The basis for such conceivability is the folk theory of law. As it is contingent which folk theory is actually adopted legal theories apply a very weak understanding of necessity. Theoretical claim that a certain feature of law is necessary has the function of explaining beliefs, forming the folk theory of law.
This compact and deep paper should not be judged on the basis of a single reading, but if you are interested in the modal status of claims about the nature of law, you will want to read it. It makes good use of David Chalmers on conceivability, but does not consider the implications of Kripke/Putnam. Not sure what I think.